Considerable effort has been devoted for developing rapidly and widely tunable wavelength laser sources for optical reflectometry, biomedical imaging, sensor interrogation, and tests and measurements. A narrow line width, wide-range and rapid tuning have been obtained by the use of an intra-cavity narrow band wavelength scanning filter. Mode-hopping-free, single-frequency operation has been demonstrated in an extended-cavity semiconductor laser by using a diffraction grating filter design. Obtaining single-frequency laser operation and ensuring mode-hop-free tuning, however, may use a complicated mechanical apparatus and can limit the maximum tuning speed. One of the fastest tuning speeds demonstrated so far has been limited less than 100 nm/s. In certain exemplary applications such as biomedical imaging, multiple-longitudinal mode operation, corresponding to an instantaneous line width as large or great than 10 GHz, may be sufficient. Such width may provide a ranging depth of a few millimeters in tissues in optical coherence tomography and a micrometer-level transverse resolution in spectrally-encoded confocal microscopy.
A line width on the order of 10 GHz can be achieved with the use of an intra-cavity tuning element (such as an acousto-optic filter, Fabry-Perot filter, and galvanometer-driven diffraction grating filter). However, the sweep frequency previously demonstrated has been less than 1 kHz limited by finite tuning speeds of the filters. Higher-speed tuning with a repetition rate greater than 15 kHz may be needed for video-rate (>30 frames/s), high-resolution optical imaging in biomedical applications.
Recent implementation of a wavelength-swept laser using polygon scanning filter has provided high-speed wavelength tuning up to 10,000 nm/ms. While the high-speed polygon based wavelength-swept light source enabled high-speed imaging as fast as 200 frames/s, wavelength tuning rate as fast as 10,000 nm/ms keeping an instantaneous linewidth narrower than 0.15 nm has already reached to the limit of the polygon based wavelength-swept laser.
Accordingly, there may be a need for new wavelength scanning filter and laser scheme for faster tuning and especially for wide wavelength tuning range and narrow instantaneous linewidth at fast tuning rate.
One of the objects of the present invention is to overcome the above-described deficiencies.